ultrasonic monitoring of concrete

7/30/2019 ultrasonic monitoring of concrete

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ACTIVE PROTECTION OF CORRODED STEEL REBAR

EMBEDDED IN FRP WRAPPED CONCRETE

A thesis submitted in partial fulfillment

of the requirement for the award of degree of

MASTER OF ENGINEERING

IN

STRUCTURES

Submitted by:

Nimrat Pal KaurRoll no:-800922014

Under the supervision of

Dr. Abhijit Mukherjee Dr. Shweta Goyal

Director Assistant Professor, CED

Thapar University Thapar University

DEPARTMENT OF CIVIL ENGINEERING

THAPAR UNIVERSITY

(Established under the section 3 of UGC Act, 1956)

PATIALA-147004 (PUNJAB)

INDIA

JULY - 2011


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ACKNOWLEDGEMENT

A dissertation cannot be completed without the help of many people who contribute directly or

indirectly through there constructive criticism in the evolution and preparation of this work. It

would not be fair on my part, if I dont say a word of thanks to all those whose sincere advice

made this period a real educative, enlightening, pleasurable and memorable one.

First of all, a special debt of gratitude is owned to my supervisors, Dr. Abhijit Mukherjee and

Dr. Shweta Goyal for their gracious efforts and keen pursuits, which has remained as a valuable

asset for the successful completion of research work. Their dynamism and diligent enthusiasm

has been highly instrumental in keeping my spirit high. The flawless and forthright suggestions

blended with an innate intelligent application have crowned my task a success.

I also like to offer my sincere thanks to all faculty members, teaching and non-teaching staff of

Civil Engineering Department (CED), and staff of central library, TU, Patiala for their

assistance. I am extremely thankful to Mr. Amarjit, Mr. Ram Simran, Mr. Surinder, and all other

manpower for helping me carry out experimental work.

I am highly obliged to Dr N.K. Verma and Mr Gurmeet of School of Material Sciences for their

support and valuable suggestions in carrying out my research work.

I would also like to thank to my parents, brothers, sisters and my friends for their constant

encouragement during the entire course of my seminar work.

NIMRAT PAL KAUR

M.E CIVIL (STRUCTURES)

ROLL NO 800922014


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ABSTRACT

Reinforced concrete is one of the most commonly used construction materials in civil

engineering but its durability problems have been obsessing people. The worst of these problems

is caused by corrosion of steel in concrete, inducing the early deterioration of concrete

infrastructures. Structural deterioration of reinforced concrete structures affected by corrosion is

a gradual process consisting of a few different phases during service life, including corrosion

initiation, concrete cracking, excessive deflection and final collapse due to loss of structural

strength.

An increasing number of concrete structures are being monitored to enhance their durability. Inthe last few decades, a number of damage detection techniques such as destructive and non

destructive techniques have been developed to analyze the changes in a structure due to

corrosion.

A new development in the repair and rehabilitation of RC systems is the use of carbon fiber

reinforced polymers (CFRP). These materials have received great attention and their applications

to structural repair and retrofit have grown significantly in recent years. CFRP fabrics are largely

employed because it offers superior performance such as resistance to corrosion, high stiffness-

to-weight ratio, high tensile strength, light weight, high durability and easy installation etc.

The CFRP wraps provide a barrier layer that is expected to impede further corrosion of steel. The

FRP sheets, when wrapped around RC members, apply confinement pressure that prevents

increase in volume due to rusting. Thus it prevents dislodging of concrete cover. Another

important advantage CFRP could be active protection of structures using CFRP wraps as anode.

The unique proposition of the present thesis is the use of FRP wraps as anode for active

protection. However, it is important to maintain high and uniform electric conductivity of the

wrap for effective protection. This thesis investigates carbon FRP (CFRP) composites for active

protection of RC structures by using non destructive monitoring techniques.


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CONTENTS

CHAPTER 1 INTRODUCTION .............................................................................................. 1-5

1.1 General .................................................................................................................................. 1

1.2 Corrosion Monitoring of RC Structures ................................................................................ 2

1.3 Frps And Monitoring of Retrofitted Elements ...................................................................... 4

1.4 Format of Thesis.................................................................................................................... 5

CHAPTER 2 CORROSION PROCESS ................................................................................ 7-14

2.1 Introduction ........................................................................................................................... 7

2.2 Causes of Rebar Corrosion .................................................................................................... 7

2.2.1 Loss of Alkanity Due To Chlorides................................................................................. 8

2.2.2 Loss of Alkanity Due To Carbonation ............................................................................ 9

2.2.3 Cracks Due To Mechanical Loading.............................................................................. 9

2.2.4 Corrosion of Rebar Due To Atmospheric Pollution ..................................................... 10

2.2.5 Moisture Pathways ....................................................................................................... 10

2.2.6 Water-Cement Ratio ..................................................................................................... 10

2.2.7 Corrosion Due To Difference In Environments ........................................................... 10

2.2.8 Low Concrete Tensile Strength..................................................................................... 11

2.2.9 Electrical Contact With Dissimilar Metals .................................................................. 11

2.3 Corrosion Mechanism of Rebar .......................................................................................... 11

2.4 Closing Remarks ................................................................................................................. 14

CHAPTER 3 NDT TECHNIQUES FOR MONITORING OF RC STRUCTURES ....... 15-21

3.1 Introduction ......................................................................................................................... 15

3.2 Importance And Need of Non-Destructive Testing ............................................................ 15


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3.3 Basic Methods For NDT Of RC Structures ........................................................................ 16

3.3.1 Half-Cell Potential Method......................................................................................... 17

3.3.2 Linear Polarization Resistance (LPR).......................................................................... 18

3.3.3 Ultrasonic Pulse Velocity Testing................................................................................ 19

3.4 Closing Remarks ................................................................................................................. 21

CHAPTER 4 FIBER REINFORCED POLYMER (FRP) AND RETROFITTING ....... 22-31

4.1 Introduction ......................................................................................................................... 22

4.2 Need of Retrofitting In RC Structures................................................................................. 22

4.3 What Are FRPs ? ................................................................................................................. 23

4.4 Suitability of FRP For Uses In Structural Engineering ....................................................... 24

4.5 Types Of FRPs .................................................................................................................... 26

4.5.1 Carbon Fibre Reinforced Polymer (CFRP) ................................................................. 27

4.5.2 Glass Fibre Reinforced Polymer (GFRP) .................................................................... 27

4.5.3 Aramid Fibre Reinforced Polymers (AFRP) ................................................................ 28

4.6 Properties of CFRP.............................................................................................................. 29

4.7 Suitability of CFRP In Civil Engineering ........................................................................... 29

4.8 Closing Remarks ................................................................................................................. 31

CHAPTER 5 LITERATURE REVIEW .............................................................................. 32-44

5.1 General ................................................................................................................................ 32

5.2 Literature Review on Electrochemical Techniques ............................................................ 32

5.3 Literature Review on Ultrasonic Techniques ...................................................................... 38

5.4 Literature Review on Monitoring of Structures Retrofitted With FRP ............................... 40

5.5 Closing Remarks ................................................................................................................. 44

CHAPTER 6 EXPERIMENTAL PROGRAMME ............................................................. 45-65


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6.1 General ................................................................................................................................ 45

6.2 Test Programme .................................................................................................................. 45

6.3 Materials Used..................................................................................................................... 47

6.3.1 Cement.......................................................................................................................... 47

6.3.2 Fine Aggregates............................................................................................................ 47

6.3.3 Coarse Aggregates ....................................................................................................... 48

6.3.4 Water............................................................................................................................ 49

6.3.5 Steel Reinforcement...................................................................................................... 49

6.3.6 CFRP Material............................................................................................................. 50

6.3.7 Adhesives ...................................................................................................................... 50

6.4 Design Of Concrete Mix ..................................................................................................... 51

6.5 Test Procedure ..................................................................................................................... 51

6.5.1 General......................................................................................................................... 51

6.5.2 Preparation And Preconditioning of Steel Bars........................................................... 52

6.5.3 Preparation of Slab Specimen ...................................................................................... 52

6.6 Corrosion Monitoring Techniques ...................................................................................... 53

6.6.1 Electrochemical Techniques......................................................................................... 53

6.6.1.1 Half cell potentialmeasurements........................................................................... 54

6.6.1.2 Linear Polarization resistance(LPR) measurements..............................................55

6.6.2 Ultrasonic Pulse Velocity Measurements ..................................................................... 56

6.7 Inducing Corrosion In Steel Rebar ...................................................................................... 59

6.8 Wrapping The Pre-Corroded Specimens............................................................................. 61

6.9 Active Protection ................................................................................................................. 62

6.10 Corroding The Wrapped Specimens ................................................................................. 63


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6.11 Closing Remarks ............................................................................................................... 65

CHAPTER 7 RESULTS AND DISCUSSIONS .................................................................. 66-79

7.1 General ................................................................................................................................ 66

7.2 Electrochemical Measurements........................................................................................... 66

7.2.1. Half Cell Measurements .............................................................................................. 66

7.2.2 Corrosion Rate By LPR Technique .............................................................................. 70

7.3 Ultrasonic measurements .................................................................................................... 72

7.4 Closing Remarks ................................................................................................................. 79

CHAPTER 8 CONCLUSIONS .................................................................................................. 81

REFERENCES ............................................................................................................................ 83


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LIST OF FIGURES

Figure No. Title Page No.

3.1 Half-cell electrical potential method 18

3.2 Linear Polarization Resistance method 19

3.3 Ultrasonic Pulse Velocity Signal 20

3.4 Principle of pulse echo method of inspection 20

3.5 Principle of through transmission of ultrasonic testing 21

4.1(a) Corrosion damage to a concrete beam

(b) FRP on a bridge structure23

4.2 (a). Externally bonded carbon fiber reinforced polymer (FRP)composites strengthen openings in concrete for new mechanical

systems in a building being adapted for a new use.

(b) Seismic retrofitting of column-beam joints of Aigaleo football

stadium in Athens, Greece, using CFRP fabrics with steel

anchorages.

25

25

4.3 Carbon fiber sheet 27

4.4 Glass fiber sheet 28

4.5 Aramid fiber sheet 28

4.6 Retrofitting of beam by FRP Sheets 30

6.1 Specimens and the Power Supplies Used to Accelerate Corrosion 46

6.2 CFRP sheet used in the experiment 50

6.3 ACM Setup Used for Electrochemical Monitoring 54

6.4 Half Cell Arrangement 55

6.5 Guard Ring Arrangement 56

6.6 Karl Deutsch Contact Type Transducers 57

6.7 Transducers attached at the Two Ends of Rebar 57

6.8 Set Up Used For Ultrasonic Investigations 58

6.9 View of Signatures As Taken On the Computer Screen 59

6.10 View of Stainless Steel Mesh 60


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6.11 Dripping With 5% NaCl Solution 60

6.12 View of Grinder 61

6.13 Applying Epoxy on Concrete 61

6.14 Placing Carbon Fiber Sheets 62

6.15 Rod Used For Smoothening 62

6.16 Setup of Beam Specimens 63

6.17 Top View of Beam Showing Terminals for Active Protection 64

7.1 Variation of half-cell potential with time (Current applied-10 mA) 66

7.2 Variation of half-cell potential with time (Current applied-30 mA) 67

7.3 Longitudinal crack along the length of rebar 68

7.4 Variation of LPR with time (Current applied-10 mA) 70

7.5 Variation of LPR with time (Current applied-30 mA ) 71

7.6Typical Signal peaks at Different Stages of corrosion subsequent

protection (0.1 MHz)73

7.7Shows Typical Signal Peaks at Different Stages of corrosion

subsequent protection (1 MHz)74

7.8(a)Peak to Peak Voltage trends of transmitted pulse (Current applied-

10 mA)

76

7.8(b) Peak Voltage trends of transmitted pulse (Current applied-10 mA) 76

7.8(c)Peak to Peak Voltage ratio trends of transmitted pulse (Current

applied-10 mA)77

7.9(a)Peak to Peak Voltage trends of transmitted pulse (Current applied-

30 mA)77

7.9(b)Peak to Peak Voltage trends of transmitted pulse (Current applied-

30 mA)78

7.9 (c)Peak to Peak Voltage trends of transmitted pulse (Current applied-

30 mA)78


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LIST OF TABLES

Table No. Title Page No.

6.1 Test Specimens with Different Level of Current 46

6.2 Physical Properties of Cement 47

6.3 Physical Properties of Fine Aggregates 48

6.4 Sieve Analysis of Fine Aggregate 48

6.5 Physical Properties of Coarse Aggregates 49

6.6 Sieve Analysis of Coarse Aggregates 49

6.7 Properties of Reinforcing Bars Used for Casting of RC Beams 50

6.8 Properties of CFRP Sheets 50

6.9 Properties of Saturant 51

6.10 The ASTM Interpretation of Half-Cell Potential Readings 55


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CHAPTER 1

INTRODUCTION

1.1 GENERAL

Reinforced concrete (RC) is an extremely popular construction material. It has proven to be

successful in terms of both structural performance and durability. Because of the nature and role

of concrete in the creation, rehabilitation and regeneration of the infrastructure system of any

country, RC plays a very important part in a nations economic development. Lack of durability

of RC structures has thus not only massive economic implications to a nations well-being, but it

is also one of the greatest threats to sustainable growth of concrete and construction industries.

Whatever the source of deterioration and the mechanism of its development, corrosion of

embedded reinforcement is recognized as the major problem affecting the durability of concrete

structures. It has been found that 40% failure of structures is on account of corrosion of

embedded steel in concrete (Sethy, 2005). Therefore corrosion control of steel reinforcement is a

subject of paramount importance. Corrosion is a form of damage which is often insidious and

hidden until striking at the worst moment of a system operation. The reason of these phenomena

is explainable with the mechanism of corrosion. When reinforcement corrodes, the corrosion

products generally occupy considerably more volume than the steel. The magnitude of this

increase in volume varies approximately 2 or 3 times the volume of the original material. As a

result, the corrosion products produce an internal stress that destroys the neighboring concrete

under tensile stress.

Maintenance and repair of reinforced concrete structures mainly due to corrosion damage is

presently one of the most significant challenges facing the concrete industry. While there are

many ways to try and prevent such damage, the optimum control method relies on an early

diagnosis of the problems. Reinforcing steel in good quality concrete does not corrode even if

sufficient moisture and oxygen are available. This is due to the spontaneous formation of a thin

protective oxide film (passive film) on the steel surface in the highly alkaline pore solution of the

concrete. When sufficient chloride ions (from deicing salts or from sea water) have penetrated to


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the reinforcement or when the pH of the pore solution drops to low values due to carbonation,

the protective film is destroyed and the reinforcing steel is depassivated.

Although corrosion of steel may not immediately affect the integrity and the ultimate load

carrying capacity of a RC structural member, it is the most complex, insidious and destructiveform of damage. Once it starts, it is almost impossible to stop the process until eventually the

safety, stability and design service life are all drastically reduced with time.

1.2 CORROSION MONITORING OF RC STRUCTURES

Waiting for visible signs of distress to appear on concrete structures is a very expensive method

of maintaining a structure. Repair cost by this stage can be astronomical (especially when costs

to take the structure off line are included). Therefore corrosion monitoring is necessary as it

allows the corrosion to be caught before its onset and opens up much more economical

maintenance options such as coating, retrofitting etc. Corrosion measurement employs a variety

of techniques to determine how corrosive the environment is and at what rate metal loss is being

experienced. Corrosion measurement is the quantitative method by which the effectiveness of

corrosion control and prevention techniques can be evaluated and provides the feedback to

enable corrosion control and prevention methods to be optimized.

Corrosion monitoring gives a complete picture of the changing condition of a structure with time

and there are several methods of monitoring the corrosion of steel reinforcement in concrete for

laboratory tests. The more common of these methods are the half cell, linear polarization and AC

impedance. The half cell method only predicts the probability of corrosion activity whereas

linear polarization and AC impedance are capable of measuring the corrosion rate that occurs in

a system. (Raharinaivo et. al (1986), Bonacci & Maleej (2000), Bertolinia et. al (2004)).

Various techniques for measuring the corrosion rate have been used to detect the corrosion at an

early stage, in order to predict residual lives and accordingly decide what preventive or repair

systems are to be applied (Song and Saraswathy, 2007). New RC structures incorporating one or

more of these new strategies are likely to have much longer durable service life than those

constructed before.


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In the last few decades, a number of damage detection techniques such as destructive and non

destructive techniques have been developed to analyze the changes in a structure due to

corrosion. Destructive techniques result in accurate measurements and gives specific

characteristics of materials by destroying the specimen. Pullout strength, mass loss and

compression test are some examples of destructive testing. Non-destructive techniques, on the

other hand, monitor the material quality without destroying the specimen. For example, the oil

and gas systems deployed in the most remote areas, often at depths never exploited before, or the

transmission pipelines traversing the harshest environments on the planet. The inspectability of

these systems is very limited and extremely costly. Therefore in many areas of modern

engineering, non-destructive evaluation (NDE) techniques have provided valuable and often

critical information for the safe operation of the most complex systems. Such usefulness has

recently been greatly enhanced by the tremendous advances in computer and communication

tools.

Non-destructive techniques such as visual inspection of the surface of the structure are simple

and cost-effective technique. However, it relies on the skill of the inspector and cannot provide

accurate information on crack depth. Moreover, cracks that have just started to appear may be

too small to be visible. Therefore, other non-destructive techniques such as radiography,

ultrasonic testing, magnetic particle testing, acoustic emission etc, are necessary to complement

visual inspection. The most common nondestructive techniques to monitor corrosion in

reinforced concrete structures are given below which are discussed in detail in Chapter 3 ( Song

and Saraswathy, 2007):

Open circuit potential Measurements Surface potential Measurements Linear Polarization Resistance Method Electrochemical Impedance Spectroscopy Half Cell Potential Measurements Ultrasonic Pulse Velocity Method Impact-echo method Acoustic Emission Infrared Thermography


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Due to the presence of damage in a structure, a number of acoustic/ultrasonic damage detection

techniques are developed to analyze the changes of linear properties. Of all the techniques used

for monitoring reinforcement corrosion, ultrasonic inspection or ultrasonic testing (UT) is one of

the most widely used NDE techniques, which is applied to measure a variety of material

characteristics and conditions. Ultrasonic examination is performed using a device that generates

an ultrasonic wave with a piezoelectric crystal at a frequency between 0.1 and 25 MHz into the

piece being examined and analyzes the return signal. The method consists in measuring the time

it takes for the signal to return and the amount and shape of that signal.

1.3 FRPs AND MONITORING OF RETROFITTED ELEMENTS

Degradation of steel reinforcements due to corrosion, cracking of concrete due to weathering,

rapidly changing traffic needs (both in terms of intensity and load levels) and recent earthquake

damages have necessitated the use of strengthening of basic structural components such as slabs,

panels, walls, beams and columns. Fiber Reinforced Plastic (FRP) composite wraps, which are

commonly used in seismic retrofits have been considered as an effective tool for repairing

corrosion damage in concrete. FRP materials have very high strength-to-weight and strength-to-

stiffness ratios. They are corrosion resistant and have a low axial coefficient of thermal

expansion. The material is ideal for retrofits because it is easy to handle, can conform to the

shape of existing elements, and can be applied quickly. Some disadvantages of FRP composites

are high initial costs, but is overcomed regarding long-term behavior of the material properties

and long-term durability.

Glass and Carbon fibre reinforced polymers (CFRP) sheets have been extensively used in

todays world for retrofitting of existing structures. These are usually bonded to the surface of

the concrete structure and are utilized to strengthen existing structures or rehabilitate structures

damaged by corrosion.

For strengthening beams, two techniques are adopted. First one is to paste FRP plates to the the

tensile face of a beam. This increases the strength of beam, deflection capacity of beam and

stiffness (load required to make unit deflection). Alternately, FRP strips can be pasted in 'U'

shape around the sides and bottom of a beam, resulting in higher shear resistance. Columns in

building can be wrapped with FRP for achieving higher strength and to provide confinement


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pressure, hence controls further formation of cracks in concrete. It also increases the load

carrying capacity of the system.

The FRP sheets are being used for repair, strengthening and retrofitting of structural components

due to their low weight, ease of handling and rapid implementation also FRP wrapped sampleshave shown substantially higher resistance to corrosion (Gadve et. al (2008), Bonacci & Maleej

(2000), Debaiky et. al (2002)). However after repair of the corroded structure, it is essential to

monitor the structure for further damage.

1.4 FORMAT OF THESIS

The main aim of the study is to monitor the corrosion behavior of RC elements by using NDT

techniques and providing active protection by retrofitting with FRP sheets.

The thesis has been divided into five chapters.

1stchapteris about general introduction about corrosion, its monitoring, FRPs and monitoring of

FRP elements.

2nd chapterexplains in detail the causes and mechanism of rebar corrosion. Also a brief

description on different types of corrosion is been given.

3

rd

chapterdeals with various NDT methods used for monitoring of RC structures. A number ofelectrochemical rebar corrosion measurement techniques available presently are reviewed to

possess with certain advantages and limitations.

4th chapterdeals with FRPs and monitoring of retrofitted elements. In this, various types of

FRPs their advantages and suitability have been discussed. Also the need for monitoring of

retrofitted elements has been mentioned.

5thchapterpresents a thorough review of literature on nondestructive monitoring techniques as

well as the Ultrasonic guided waves for monitoring rebar corrosion in concrete. Also a thorough

review of literature on Fiber Reinforced Plastic (FRP) composite wraps for corrosion protection

is also presented.


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6thchapterdeals with the experimental programme wherein all tests, procedures and measures to

be followed during experiments are explained in detail.

7thchapterdeals with results and discussions where findings of the experimental programme are

explained in detail.

8thchapteris the concluding chapter.


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CHAPTER 2

CORROSION PROCESS

This chapter discusses the basics of corrosion and how they apply to steel in concrete.

2.1 INTRODUCTION

Given the widespread use of reinforced concrete in infrastructure, understanding the corrosion of

rebar is of major importance. Corrosion of steel reinforcement is the most common durability

problem of reinforced concrete structures. Steel in concrete is normally protected from corrosion

by a passive film of iron oxides on the steel surface resulting from the natural alkaline

environment of the concrete. The passive film is chemically stable in the absence of carbonation

and chloride ions (Bentur et. al (1997), Broomfield (1997)). The ingress of chloride ions (Cl) to

the level of the steel reinforcing bars destroys the passive film and initiates corrosion. This

makes reinforced concrete structures in coastal areas and/or marine environments vulnerable to

damage by corrosion of steel reinforcement. Reinforced concrete infrastructures located in cold

environments are also susceptible to corrosion damage due to the use of deicing salts. Once

corrosion is initiated, electrochemical reactions occur, leading to the formation of expansive

corrosion products that create tensile stresses in the concrete surrounding the corroding steel

reinforcing bar. This results in concrete cracking and spalling, which aggravates the progressive

damage, thus affecting the durability of the structure.

In this chapter the basic mechanisms of corrosion have been discussed.

2.2 CAUSES OF REBAR CORROSION

It is well known that if bright steel is left unprotected in the atmosphere a brown oxide rust

quickly forms and will continue to grow until a scale flakes from the surface. In the concrete

structures, reinforcing steel-bars (rebars) normally do not corrode because of a passive film

formed on the surface of rebar in concrete of high pH. When chloride concentration at the level

of rebar in concrete, however, exceeds the threshold value for corrosion, the passive film is

destroyed and corrosion is initiated in rebar. The electro-chemical reaction continues with


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supplying oxygen and water. Then, due to expansion of corrosion products, corrosion-induced

cracks are generated in concrete.

Sound concrete is an ideal environment for steel but the increased use of deicing salts and the

increased concentration of carbon dioxide in modern environments principally due to industrialpollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this

material. The scale of this problem has reached alarming proportions in various parts of the

world.

Carbonation of concrete or penetration of chlorides into the concrete, are the major causes of

reinforcement corrosion. Chlorides in concrete either penetrate from the surrounding chloride-

bearing environment (such as moisture, oxygen, humidity, temperature, bacterial attack, stray

currents, etc.) or contribute from the concrete ingredients (such as concrete quality, w/c ratio,

cement content, impurities in the concrete ingredients, presence of surface cracks, etc).

The assessment of the causes and extent of corrosion is carried out using various electrochemical

techniques (Broomfield, 2006). Prediction of the remaining service life of a corroding reinforced

concrete infrastructure is done with the help of empirical models and experimental methods

(Weyers, 1998).

Following are the two most common contributing factors leading to reinforcement corrosion:

(i) Localized breakdown of the passive film on the steel by chloride ions called chloride attack.

(ii) General breakdown of passivity by neutralization of the concrete, predominantly by reaction

with atmospheric carbon dioxide called carbonation.

These major factors along with various other factors that lead to rebar corrosion are explained in

detail in the following sections.

[

2.2.1 Loss of Alkanity due to Chlorides

The passivity provided by the alkaline conditions can also be destroyed by the presence of

chloride ions, even though a high level of alkalinity remains in the concrete. The chloride ion can

locally de-passivate the metal and promote active metal dissolution. Chlorides react with the

calcium aluminate and calcium aluminoferrite in the concrete to form insoluble calcium

chloroaluminates and calcium chloroferrites in which the chloride is bound in non-active form.

However, the reaction is never complete and some active soluble chloride always remains in


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equilibrium in the aqueous phase in the concrete. It is this chloride in solution that is free to

promote corrosion of the steel. At low levels of chloride in the aqueous phase, the rate of

corrosion is very small, but higher concentration increases the risks of corrosion.

2.2.2 Loss of Alkanity due to Carbonation

Carbonation is a result of the interaction of carbon dioxide gas in the atmosphere with the

alkaline hydroxides in the concrete. Due to the high alkalinity of the concrete pore water, the

steel reinforcing bars are passivated by an iron oxide film (Fe2O3) that protects the steel. The

passivating layer of hydrated iron oxide is 220 nm thick. At this pH value a passive film forms

on the steel that reduces the rate of corrosion to a very low and harmless value.

Concrete is permeable and allows the slow ingress of the atmosphere; the acidic gases react with

the alkalis (usually calcium, sodium and potassium hydroxides), neutralizing them by forming

carbonates and sulphates, and at the same time reducing the pH value. If the carbonated front

penetrates sufficiently deeply into the concrete to intersect with the concrete reinforcement

interface, protection is lost and, since both oxygen and moisture are available, the steel is likely

to corrode. The extent of the advance of the carbonation front depends, to a considerable extent,

on the porosity and permeability of the concrete and on the conditions of the exposure.

In the case of carbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali

according to the generalized reaction,

Ca(OH)2 + CO2 CaCO3 + H2O 2.1

It consumes alkalinity and reduces pore water pH to the 89 range, where steel is no longer

passive.

2.2.3 Cracks due to Mechanical Loading

Cracks in concrete formed as a result of tensile loading, shrinkage, frost attack or other factors

allows the ingress of water and oxygen from atmosphere and provide a zone from which the

carbonation front can develop. If the crack reaches the surface of rebar the protection can be lost.

Due to formation of cracks, debonding of steel and concrete occurs to some extent on each side

of the crack, thus removing the alkaline environment and so destroying the protection in the

vicinity of the debonding.


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2.2.4 Corrosion of Rebar due to Atmospheric Pollution

Most of the times steel reinforcement is exposed to the atmosphere during transportation and

storage in the building sites for a long period before their installation in the concrete structures.

At any of those stages, steel rebars can be contaminated by chloride ions from windblown salt.

This fact leads to the formation of corrosion products on their surface.

2.2.5 Moisture Pathways

If the surface of the concrete is subjected to long-term wetting, the water will eventually reach

the level of the reinforcement, either through diffusion through the porous structure of the

concrete, or by traveling along cracks in the concrete. Concrete roof decks, by their nature, are

meant to be protected from moisture. However, the presence of moisture on roofing systems may

result from failure of the roofing membrane, poor detailing of drainage facilities, or lack of

maintenance of drainage facilities.

2.2.6 Water-Cement Ratio

Concrete placed with a high water-cement ratio, is more porous due to the presence of excess

water in the plastic concrete. The porosity increases the rate of diffusion of water and electrolytes

through the concrete and makes the concrete more susceptible to cracking.

2.2.7 Corrosion due to Difference in Environments

Corrosion occurs when two different metals, or metals in different environments, are electrically

connected in a moist or damp concrete. This will occur when:

1. Steel reinforcement is in contact with an aluminium conduit.

2. Concrete pore water composition varies between adjacent or along reinforcing bars.

3. There is a variation in alloy composition between or along reinforcing bars.

4. There is a variation in residual/applied stress along or between reinforcing bars.

The aggressiveness of environmental conditions is exacerbated by deicing salts spread on roads

and pavements. Deicing salts not only increase pressures within the concrete, but also diminish

its ability to withstand them. The application of de-icing agents to a concrete surface covered

with ice will cause a substantial drop in temperature at the concrete surface during thawing of the

ice. The difference in temperature between the surface area and the interior of the concrete gives


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rise to a state of internal stresses likely to induce cracking in the region of the outer layer of the

concrete.

2.2.8 Low Concrete Tensile Strength

Concrete with low tensile strength facilitates corrosion damage in two ways. First, the concrete

develops tension or shrinkage cracks more easily, admitting moisture and oxygen, and in some

cases chlorides to the level of the reinforcement. Second, the concrete is more susceptible to

developing cracks at the point when the reinforcement begins to corrode.

2.2.9 Electrical Contact with dissimilar metals

Dissimilar metals in contact initiate a flow of electrons that promotes the corrosion of one or the

other, by a process known as galvanic corrosion. When two dissimilar metals are in contact with

each other, the more active metal will induce corrosion on the less active metal. Such corrosion

may induce cracking and damage in the concrete.

2.3 CORROSION MECHANISM OF REBAR

The corrosion of steel reinforcing bars is an electrochemical process that requires a flow of

electric current and several chemical reactions.

The three essential components of a galvanic corrosion cell are:

Anode Cathode Electrolyte

The anode is the location on a steel reinforcing bar where corrosion is taking place and metal is

being lost. At the anode, iron atoms lose electrons to become iron ions (Fe+2

). This oxidation

reaction is referred to as the anodic reaction. The cathode is the location on a steelreinforcing bar

where metal is not consumed. At the cathode, oxygen in the presence of water, accepts electronsto form hydroxyl ions (OH

-). This reduction reaction is referred to as the cathodic reaction. The

electrolyte is the medium that facilitates the flow of electrons (electric current) between the

anode and the cathode. Concrete, when exposed to wet and dry cycles, has sufficient

conductivity to serve as an electrolyte. Fig. 2.1 illustrates the corrosion cell for a steel reinforcing

bar embedded in concrete where the anode and the cathode are on the same steel reinforcing bar.


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Fig. 2.1 Corrosion Cell in Reinforced Concrete (Gadve et al. 2010)

The corrosion of steel in concrete in the presence of oxygen but without chlorides takes place in

several steps:

At the anode, iron is oxidized to the ferrous state and releases electrons

(2.2)

These electrons migrate to the cathode where they combine with water and oxygen to form

hydroxyl ions

(2.3)

The hydroxyl ions combine with the ferrous ions to form ferrous hydroxide

(2.4)

In the presence of water and oxygen, the ferrous hydroxide is further oxidized to form Fe2O3

(2.5)

Both the anodic and cathodic reactions are necessary for the corrosion process to occur and they

need to take place simultaneously. The anode and cathode can be located next to each other or

can be separated. When they are located next to each other, i.e., on microscopic scale, the

resulting corrosion cell is referred to as a microcell. When they are separated by some finite

distance, the resulting corrosion cell is referred to as macro cell as illustrated in Fig. 2.2.

OHOOHe 2

2

12

22

2

2 OHFeOHFe

OHOFeOHFe

OHFeOHOOHFe

2323

3222

2.2

44

eFeFe 2


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Corrosion of steel reinforcing bars embedded in concrete may be due to a combination of macro

cells and micro cells.

Fig. 2.2 Micro cell and Macro cell (Gadve et al. 2010)

The corrosion products resulting from the corrosion of steel reinforcing bars occupy a volume

five to ten times that of the original steel. This increase in volume induces stresses in the

concrete that result in cracks, delaminations and spalls. If left untreated, the process continues

which further accelerates the corrosion process by providing an easy pathway for water and

chlorides to reach the steel until the concrete becomes structurally unsound as shown in Fig 2.3.

Fig.2.3 Cracking and spalling of concrete due to penetration of chloride ions

The minimum chloride ion concentration needed to initiate corrosion of steel reinforcing bars is

also called the chloride threshold. Although the concept of chloride threshold is generally

accepted, there is little agreement on what the threshold value is. Several factors influence the

chloride threshold value: the composition of the concrete (resistivity), the amount of moisture

present, and the atmospheric conditions (temperature and humidity). The threshold concentration


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depends on the pH level and the concentration of oxygen. When chlorides are uniformly

distributed, higher concentrations are needed to initiate corrosion. Regardless of what

concentration of chloride ions is needed to initiate corrosion, an increase in the chloride ion

concentration increases the probability that corrosion of steel reinforcing bars will occur.

2.4 CLOSING REMARKS

This chapter discusses the theory of corrosion mechanism and causes of rebar. The various

factors and effect of corrosion process are well discussed. It is concluded that the two most

important causes of rebar are the ingress of chloride ions and carbon dioxide to the steel surface.

To obtain maximum information about the corrosion state of rebar in a particular structure,

various non destructive monitoring techniques are necessary so as to provide an effective repair

or rehabilitation system such as coating, retrofitting, corrosion inhibitors etc. Next chapter

provides information regarding various non destructive monitoring techniques in detail.


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CHAPTER 3

NDT TECHNIQUES FOR MONITORING OF RC STRUCTURES

3.1 INTRODUCTION

Concrete structures that are subjected to repeated service loads, weathering or chemical attack

may display surface-breaking cracks. These cracks may eventually lead to failure of the structure

as they extend from the surface into the material, or take a role in the corrosion of reinforcement

in concrete (Broomfield, 2006). It is therefore essential to be able to assess the condition of

concrete structures, and more specifically, to estimate the depth of surface breaking cracks. Non-

destructive techniques (NDT) are suitable candidates to reach this goal as compared to partially

destructive tests because they allow for in-situ inspection with high spatial resolution, whereas

tests performed on drilled cores may only be performed on a limited number of measurement

points. Also, NDT will not affect the integrity of structure, as is possible with drilling cores.

3.2 IMPORTANCE AND NEED OF NON-DESTRUCTIVE TESTING

It is often necessary to test concrete structures after the concrete has hardened to determine

whether the structure is suitable for its designed use. Ideally such testing should be done withoutdamaging the element. The tests available for checking integrity of the element range from the

completely non-destructive, where there is no damage to the element, through those where the

concrete surface is slightly damaged, to partially destructive tests, such as core tests and pullout

and pull off tests, where the surface has to be repaired after the test.

The corrosion of reinforcements has resulted to be one of the most frequent causes of premature

failure of RC element. Corrosion is an insidious process, often difficult to recognize until

deterioration is well advanced. When left unchecked, corrosion will propagate in the structureand ultimately lead to damage. Monitoring the corrosion rate, assuming the uniform corrosion

and the loss in diameter decreases linear with the corrosion rate, allows calculating the remaining

load carrying capacity and the safety of the structure. There are several methods of measuring

true, instantaneous rate of corrosion, based on non-destructive testing. Non-destructive testing

can be applied to both old and new structures. For new structures, the principal applications are


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likely to be for quality control or the resolution of doubts about the quality of materials or

construction. The testing of existing structures is usually related to an assessment of structural

integrity or adequacy. Non-destructive testing can be used in the situations as a preliminary to

subsequent coring. Typical situations where non-destructive testing may be useful are, as

follows:

quality control of pre-cast units or construction in situ. removing uncertainties about the acceptability of the material supplied owing to apparent

non-compliance with specifications.

confirming or negating doubts concerning the workmanship involved in batching,mixing, placing, compacting or curing of concrete.

monitoring of strength development in relation to formwork removal, prestressing, loadapplication or similar purposes.

location and determination of the extent of cracks, voids, honeycombing and similardefects within a concrete structure.

determining the concrete uniformity, possibly preliminary to core cutting, load testing orother more expensive or disruptive tests.

determining the position and condition of reinforcement. increasing the confidence level of a smaller number of destructive tests.

3.3 BASIC METHODS FOR NDT OF RC STRUCTURES

Although a number of different non-destructive testing methods have been developed, but the

common methods with some typical applications that have been adopted for non-destructive

testing of RC strctures are explained in the following sections.

Visual inspection, which is an essential precursor to any intended non-destructive test.An experienced civil or structural engineer may be able to establish the possible cause(s)

of damage to a concrete structure and hence identify which of the various NDT methods

available could be most useful for any further investigation of the problem.

Half-cell electricalpotential method, used to detect the corrosion potential of reinforcingbars in concrete.


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Linear Polarization Resistance (LPR), used to estimate the corrosion rate of steel inconcrete.

Schmidt/rebound hammer test, used to evaluate the surface hardness of concrete. Carbonation depth measurement test, used to determine whether moisture has reached

the depth of the reinforcing bars and hence corrosion may be occurring.

Permeability test, used to measure the flow of water through the concrete. Penetration resistance or Windsor probe test, used to measure the surface hardness

and hence the strength of the surface and near surface layers of the concrete.

Covermeter testing, used to measure the distance of steel reinforcing bars beneath thesurface of the concrete and also possibly to measure the diameter of the reinforcing bars.

Radiographic testing used to detect voids in the concrete. Ultrasonic pulse velocity testing, mainly used to measure the sound velocity of concrete

and hence the compressive strength of the concrete.

Impact echo testing, used to detect voids, delamination and other anomalies in concrete. Ground penetrating radar or impulse radar testing, used to detect the position of

reinforcing bars or stressing ducts.

Infrared thermography, used to detect voids, delamination and other anomalies inconcrete and also detect water entry points in buildings.

3.3.1Half-cell electrical potential method

It is used to detect the corrosion potential of reinforcing bars in concrete. The method of half-cell

potential measurements normally involves measuring the potential of an embedded reinforcing

bar relative to a reference half-cell placed on the concrete surface. The half-cell is usually a

copper/copper sulphate or silver/silver chloride cell but other combinations are also possible.


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Fig-3.1 Half-cell electrical potential method (Song and Saraswathy, 2007)

The concrete functions as an electrolyte and the risk of corrosion of the reinforcement in the

immediate region of the test location may be related empirically to the measured potentialdifference. ASTM C876 - 91 gives a Standard Test Method for Half-Cell Potentials of uncoated

reinforcing steel in concrete. The limitation of this method is that it cannot indicate the actual

corrosion rate and it may also require to drill a small hole to enable electrical contact with the

reinforcement in the member under examination.

The ASTM interpretation of half-cell potential readings (SCE) is as follows:

Open circuit potential (OCP) values Corrosion condition

< -426 Mv Severe corrosion, corrosion induced cracking may occur

< -276 Mv High risk, 90% probability of corrosion

-126 to -275 Mv Intermediate risk, corrosion activity in uncertain

0 to -125 mV Low risk, 10% probability of corrosion

3.3.2Linear Polarization Resistance (LPR)

It is used to estimate the corrosion rate of steel in concrete. LPR has been widely used in the

laboratory and is now being applied in structures. The basic principle of LPR is to measure the

corrosion current which gives an indication of how quickly a known area of steel is corroding.

The amount of steel loss during the corrosion process can be measured to a certain degree of

accuracy by means of the measurement of the electrical current generated by anodic reaction and

consumed by the cathodic reaction. Fig 3.2 shows the LPR arrangement.


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Fig-3.2 Linear Polarization Resistance method (Song and Saraswathy, 2007)

There is a direct relationship between the measured corrosion current and the mass of steel

consumed by Faradays Law. Corrosion current can be derived indirectly through half-cell

potential measurement through the following expression given by Stern-Geary:

Icorr=

(3.1)

WhereIcorris the change in current (A /cm2),

B is a constant relating to the electrochemical characteristics of steel in concrete,

Rp is the polarization resistance expressed asRp =

It deserves emphasis that this linear relationship is only valid when the potential change is kept

less than 20mV. LPR measurement is very useful in finding the true condition of corrosion in a

structure although it is slow compared with the half-cell potential test.

3.3.3Ultrasonic pulse velocity testing

It is mainly used to measure the sound velocity of concrete and hence the compressive strength

of the concrete. A pulse of longitudinal vibrations is produced by an electro-acoustical

transducer, which is held in contact with one surface of the concrete under test. Ultrasonic

examination is performed using a device that generates an ultrasonic wave with a piezoelectric

crystal at a frequency between 0.1 and 25 MHz into the piece being examined and analyzes the

return signal. The method is based on the measurement of time it takes for the signal to return

and the amount and shape of that signal as shown in Fig.3.3.


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Fig. 3.3 Ultrasonic Pulse Velocity Signal

The two most common methods of ultrasonic testing are:-

1. Pulse echo method

In the pulse-echo method, a piezoelectric transducer with its longitudinal axis located

perpendicular to and mounted on or near the surface of the test material is used to transmit and

receive ultrasonic energy. The ultrasonic waves are reflected by the opposite face of the material

or by discontinuities, layers, voids, or inclusions in the material, and received by the same

transducer where the reflected energy is converted into an electrical signal. Fig.3.4 shows the

principle of pulse echo method of inspection. The electrical signal is computer processed for

display on a video monitor or TV screen. The display can show the relative thickness of the

material, depth into the material where flaws are located, (with proper scanning hardware and

software), and where the flaws are located in the X-Y plane.

Fig.3.4 Principle of pulse echo method of inspection (Vermani et al. 2008)


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Fig.3.5 Principle of through transmission of ultrasonic testing (Vermani et al. 2008)

2. Through transmission method

In the through-transmission method, an ultrasonic transmitter is used on one side of the material

while a detector is placed on the opposite side. One unit acts as transmitter and the other unit as

receiver. The beam from the transmitter T travels through the material to its opposite surface

where the receiving transducer R is placed. Fig 3.5 shows theprinciple of through transmission

method of testing. Scanning of the material using this method will result in the location of

defects, flaws and inclusions in the X-Y plane.

3.4 CLOSING REMARKS

In this chapter various NDT methods for corrosion monitoring of RC structures are reviewed ans

it is observed that each method possess certain advantages and limitations. It is observed that

non-destructive techniques can be used effectively for investigation and evaluating the actual

condition of the structures but the choice of a particular NDT method depends upon the property

of concrete to be observed such as strength, corrosion, crack monitoring etc. In the present study

half cell potential, LPR and Ultrasonic pulse velocity measurements are taken in order to study

the effect of corrosion on steel rebar.


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CHAPTER 4

FIBRE REINFORCED POLYMER (FRP) AND RETROFITTING

4.1 INTRODUCTION

Retrofitting of existing infrastructure is bound to increase all over the world. This is because of

deterioration of structural strength of existing infrastructure (due to age and environmental

attacks),up-grading of various design codes(due to better understanding of various design

concepts in due course of time) and higher load carrying capacity demand (due to present day

increased service needs) etc. For years, civil engineers have been in search for alternatives to

steels and alloys to combat the high costs of repair and maintenance of structures damaged by

corrosion and heavy use. Since 1940s, composite materials, formed by the combination of two or

more distinct materials in a microscopic scale, have gained increasing popularity in the

engineering field. Fiber Reinforced Polymer (FRP) is a relatively new class of composite

material manufactured from fibers and resins and has proven efficient and economical for the

development and repair of new and deteriorating structures in civil engineering. Wide spread

utilization of FRPs in construction is hampered by lack of long-term durability and performance

data in tropical environment.

4.2 NEED OF RETROFITTING IN RC STRUCTURES

Over the last 50 years, the strengths of various types of concrete have increased from the low

levels of 15-20 MPa to values in the range of 40-70 MPa. Retrofitting of concrete structures has

become an increasingly dominant use of the material in structural engineering applications. Such

uses include increasing the load capacity of existing structures (such as existing parking garages)

that were designed to tolerate far lower service loads. Other uses include seismic retrofitting, and

repair of damaged concrete structures. Repair and rehabilitation work for concrete structures can

broadly be classified into two categories:

repair in which damage due to deterioration and cracking is corrected to restore theoriginal structural shape, and


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repair which is necessary to strengthen the structural capacity of members whose loadcarrying capacity is either inadequate or whose strength has been severely impaired due

to sustained damage.

Degradation of steel reinforcements due to corrosion, cracking of concrete due to weathering,

rapidly changing traffic needs (both in terms of intensity and load levels) and recent earthquake

damages have necessitated the use of strengthening of basic structural components such as slabs,

panels, walls, beams and columns. Various researchers have reported that retrofitting by FRP

wraps slows down the rate of corrosion thereby preventing the structure from damage (Kutarba

and Hamilton (2007),Spainhour et al. (2008),Bonacci and Maalej (2000)).

(a) Corrosion damage to a concrete beam (b) FRP on a bridge structure

Fig.4.1 (a) Corrosion damage to a concrete beam, (b) FRP on a bridge structure (Kutarba

and Hami lton (2007).

Retrofitting of reinforced concrete beams or columns in which the steel reinforcement has been

damaged by corrosion is one area of particular interest in marine environments and in locations

where deicing salts are used. Figure 4.1 (a) shows the two-fold result of severe corrosion

damage to a concrete beam. Figure 4.1 (b) shows the application of FRP on a bridge structure.

The bridge girder was strengthened for flexure and shear using a wet lay-up system (Kutarba and

Hamilton (2007)).

4.3 WHAT ARE FRPs ?

Fibre-reinforced polymer commonly known as FRPs represents a class of materials that falls into

a category referred to as composite materials. Composite materials consist of two or more

materials that retain their respective chemical and physical characteristics when combined

together. FRPs are commonly used in the aerospace, automotive, marine, and construction
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industries. Fibre-reinforced polymer is a composite material made of a polymer matrix

reinforced with fibres. The fibres are usually glass, carbon, or aramid, while the polymer is

usually an epoxy, vinylesterorpolyesterthermosetting plastic. Fibres can be formed from a wide

range of amorphous and crystalline materials but in the construction industry the three fibres

which are generally used in structural systems are (Hollaway and Head, 2001).

the glass fibre (the E-glass fibre, the S-glass fibre and the Z-glass fibre), the aramid fibre (the aromatic polyamides, Kevlar 49 fibre) and the carbon fibre (the ultra high-modulus fibre, the high-modulus fibre and the high-

strength fibre).

The primary function of fibre reinforcement is to carry load along the length of the fibre and to

provide strength and stiffness in one direction. FRP composites are different from traditional

construction materials like steel or aluminium. FRP composites are anisotropic (properties

apparent in the direction of applied load) whereas steel or aluminium is isotropic (uniform

properties in all directions, independent of applied load). Therefore FRP composites properties

are directional, i.e. the best mechanical properties are in the direction of the fibre placement.

However like other materials FRPs are also bounded to have defects and can decay during the

service life. Defects in composites may be due to several factors including improper design,

fabrication and manufacturing and air voids. During manufacturing of FRPs due to poor

workmanship some defects may occur which may adversely affect the fiber matrix adhesion

properties, resulting in debonding at the fiber/matrix interfaces, micro-cracking in the matrix,

fiber fragmentation, continuous cracks and several other phenomena that may actually degrade

the mechanical property of the composites. So assessment of cracks in FRP and monitoring of

delamination in FRP is also required.

4.4 SUITABILITY OF FRP FOR USES IN STRUCTURAL ENGINEERING

Externally bonded fibre-reinforced polymers (FRPs) in the form of continuous carbon (C), glass

(G) or aramid (A) fibres bonded together in a matrix made of epoxy, vinylester or polyester, have

been employed extensively throughout the world in numerous rehabilitation applications of

reinforced concrete or masonry structures. The high strength-to-weight ratio, immunity to

corrosion and easy handling and installation has made FRP jackets the material of choice in an
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increasingly large number of projects where increased strength or inelastic deformation capacity

must be achieved for seismic retrofitting, despite the relatively high material costs.

Since its first applications in Europe and Japan in the 1980s, use of bonded repair and retrofit of

concrete structures with fiber reinforced polymer (FRP) systems has progressively increased tothe extent that today it counts for at least 25 Innovative Bridge Research and Construction

(IBRC) projects in the United States, in addition to numerous projects independently undertaken

by state departments of transportation (DOTs) and countries.

Fibre reinforced polymer (FRP) composite system into construction is a promising alternative for

the rehabilitation of weakened and deficient concrete members. Fibre Reinforced Polymer

Composites, originally developed for the aerospace industry, show immense potential as a

material that can be used in civil infrastructure. The strength properties of FRPs collectively

make up one of the primary reasons for which civil engineers select them in the design of

structures. A material's strength is governed by its ability to sustain a load without excessive

deformation or failure. When an FRP specimen is tested in axial tension, the applied force per

unit cross-sectional area (stress) is proportional to the ratio of change in a specimen's length to its

original length (strain). When the applied load is removed, FRP returns to its original shape or

length. In other words, FRP responds linear-elastically to axial stress.

Among FRPs high strength properties, the most relevant features include

Excellent durability and corrosion resistance. High strength-to-weight ratio. A member composed of FRP can support larger live loads since its dead weight does not

contribute significantly to the loads that it must bear.

Ease of installation. Versatility.

Anti-seismic behavior. Electromagnetic neutrality. Excellent fatigue behavior and fire resistance.

One of the most common uses for FRP involves the repair, rehabilitation and retrofitting of

damaged or deteriorating structures as shown in Fig. 4.2 (a), (b). Several companies across the


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world are beginning to wrap damaged bridge piers to prevent collapse and steel-reinforced

columns to improve the structural integrity and to prevent buckling of the reinforcement.

Architects have also discovered many applications for which FRP can be used. These include

structures such as siding/cladding, roofing, flooring and partitions.

(a) (b)

Fig.4.2 (a). Externally bonded carbon fiber reinforced polymer (FRP) composites

strengthen openings in concrete for new mechanical systems in a building being adapted

for a new use.

(b) Seismic retrofitting of column-beam joints of Aigaleo football stadium in Athens,

Greece, using CFRP fabrics with steel anchorages.

4.5 TYPES OF FRPs

In civil engineering three types of fibres dominate. These are carbon, glass, and aramid fibres

and the composite is often named by the reinforcing fibre, e.g. CFRP for Carbon Fibre

Reinforced Polymer. They have different properties. For strengthening purposes carbon fibres

are the most suitable. All fibres have generally higher stress capacity than the ordinary steel and

are linear elastic until failure. The most important properties that differ between the fibre types

are stiffness and tensile strain.
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4.5.1Carbon Fibre Reinforced Polymer (CFRP)

Carbon fibrereinforced polymer, is a material consisting of extremely thin fibers about 0.005

0.010 mm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded

together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber.

The crystal alignment makes the fiber very strong for its size. Several thousand carbon fibers are

twisted together to form a yarn, which may be used by itself or woven into a fabric (Fig.4.3).

Carbon fiber are combined with a plastic resin and wound or molded to form composite

materials such as carbon fiber reinforced plastic to provide a high strength-to-weight ratio

material. The properties of carbon fiber such as high tensile strength, low weight, and low

thermal expansion make it very popular in aerospace, civil engineering, military, and

motorsports, along with other competition sports. However, it is relatively expensive when

compared to similar materials such as fiberglass or plastic.

Fig.4.3 Carbon fiber sheet

4.5.2 Glass Fibre Reinforced Polymer (GFRP)

GFRP is a lightweight, strong material with very many uses, including boats, automobiles, water

tanks, roofing, pipes and cladding. The plastic matrix may be epoxy, a thermosetting plastic

(most often polyesterorvinylester) orthermoplastic. Glass fibres are basically made by mixing

silica sand, limestone, folic acid and other minor ingredients. The mix is heated until it melts atabout 1260C. The molten glass is then allowed to flow through fine holes in a platinum plate.

The glass strands are cooled, gathered and wound. The fibres are drawn to increase the

directional strength. The fibres are then woven into various forms for use in composites

(Fig.4.4). GFRP sheets are also being increasingly used in rehabilitation and retrofitting of
http://en.wikipedia.org/wiki/Fibershttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Yarnhttp://en.wikipedia.org/wiki/Wovenhttp://en.wikipedia.org/wiki/Plastichttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Carbon_fiber_reinforced_plastichttp://en.wikipedia.org/wiki/Epoxyhttp://en.wikipedia.org/wiki/Thermosetting_plastichttp://en.wikipedia.org/wiki/Polyesterhttp://en.wikipedia.org/wiki/Vinylesterhttp://en.wikipedia.org/wiki/Thermoplastichttp://en.wikipedia.org/wiki/Thermoplastichttp://en.wikipedia.org/wiki/Vinylesterhttp://en.wikipedia.org/wiki/Polyesterhttp://en.wikipedia.org/wiki/Thermosetting_plastichttp://en.wikipedia.org/wiki/Epoxyhttp://en.wikipedia.org/wiki/Carbon_fiber_reinforced_plastichttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Plastichttp://en.wikipedia.org/wiki/Wovenhttp://en.wikipedia.org/wiki/Yarnhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Fibers


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concrete structures as an alternative to steel in concrete due to their high strength-to-weight ratio,

corrosion resistance, ease of handling , temperature resistance and application at site.

Fig.4.4 Glass fiber sheet

4.5.3 Aramid Fibre Reinforced Polymers (AFRP)

Aramid is the short form for aromatic polyamide. Aramid fibers are most commonly known

Kevlar, Nomex and Technora. Aramids are generally prepared by the reaction between an amine

group and a carboxylic acid halide group, commonly this occurs when an aromatic polyamide is

spun from a liquid concentration of sulfuric acid into a crystallized fibre. Fibres are then spun

into larger threads in order to weave into large ropes or woven fabrics (Fig. 4.5). Aramid has

high fracture energy and is therefore used for helmets and bullet-proof garments. Aramid fibres

are sensitive to elevated temperatures, moisture and ultraviolet radiation and therefore not widely

used in civil engineering applications.

Fig.4.5 Aramid fiber sheet


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4.6Properties of CFRP

Carbon fibre reinforced polymer (CFRP) is alkali resistant. Carbon fibre reinforced polymers (CFRP) are resistant to corrosion; hence they are used

for corrosion control and rehabilitation of reinforced concrete structures. Carbon fibre reinforced polymer composite has low thermal conductivity. CFRP composites have high strength to weight ratio and hence it eliminates requirements

of heavy construction equipment and supporting structures.

CFRP composites are available in rolls of very long length. Therefore, they need veryfew joints, avoiding laps and splices, and its transportation is also very easy.

CFRP composites have a short curing time. Therefore, the application takes a shortertime. This reduces the project duration and down time of the structure to a great extent.

Application of CFRP composites does not require bulky and dusty materials in a largequantity; therefore, the site remains tidier.

CFRP composites possess high ultimate strain; therefore, they offer ductility to thestructure and they are suitable forearthquake resistant applications.

CFRP composites have high fatigue resistance. So they do not degrade, which easilyalleviates the requirement of frequent maintenance.

CFRP composites are bad conductor of electricity and are non-magnetic.

4.7 Suitability of CFRP in Civil Engineering

The main impetus for development of carbon fibres has come from the aerospace industry with

its need for a material with combination of high strength, high stiffness and low weight.

Recently, civil engineers and construction industry have begun to realize that this material

(CFRP) have potential to provide remedies for many problems associated with the deterioration

and strengthening of infrastructure. Effective use of carbon fibre reinforced polymer could

significantly increase the life ofstructures, minimizing the maintenance requirements.

Studied in an academic context as to its potential benefits in construction, it has also proved itself

cost-effective in a number of field applications strengthening concrete, masonry, steel, cast iron,

and timber structures.
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Civil structural applications of fiber reinforced polymer (FRP) composites include both new

constructions as well as existing structures. CFRP is widely used to strengthen concrete

structures that have lost reinforcing steel mass due to corrosion and concrete deterioration

because CFRP possess good rigidity, high strength, low density, corrosion resistance, vibration

resistance, high ultimate strain, high fatigue resistance, and low thermal conductivity. In addition

the FRP materials that are electrically conductive can be designed to offer active protection as

well. The conductive FRP wrap-around can be used as anodes and the reinforcements can act as

cathode to impede the corrosion of steel.

Carbon fibre reinforced polymer (CFRP) is currently used worldwide to retrofit and repair

structurally deficient infrastructures such as bridges and buildings. Retrofitting has become the

increasingly dominant use of the material in civil engineering, and applications includeincreasing the load capacity of old structures (such as bridges) that were designed to tolerate far

lower service loads than they are experiencing today, seismic retrofitting, and repair of damaged

structures. Fig.4.6 shows retrofitting by FRP sheets. When reinforced concrete (RC) members

are strengthened with externally bonded CFRP, the bond between the CFRP and RC substrate

significantly affects the members load carrying capacity.

Fig.4.6 Retrofitting of beam by FRP Sheets

CFRP can also be applied to enhance shear strength of reinforced concrete by wrapping fabrics

or fibers around the section to be strengthened. Wrapping around sections (such as bridge or

building columns) can also enhance the ductility of the section, greatly increasing the resistance
http://theconstructor.org/concrete/fibre-reinforced-polymer/1583/http://theconstructor.org/category/structures/bridges/http://en.wikipedia.org/wiki/Bridgehttp://schnellcontractors.com/wp/wp-content/gallery/carbon_fiber/carbon-fiber-002.jpghttp://schnellcontractors.com/wp/wp-content/gallery/carbon_fiber/carbon-fiber-001.jpghttp://en.wikipedia.org/wiki/Bridgehttp://theconstructor.org/category/structures/bridges/http://theconstructor.org/concrete/fibre-reinforced-polymer/1583/


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to collapse under earthquake loading. Such 'seismic retrofit' is the major application in

earthquake-prone areas, since it is much more economic than alternative methods. If a column is

circular (or nearly so) an increase in axial capacity is also achieved by wrapping. In this

application, the confinement of the CFRP wrap enhances the compressive strength of the

concrete. The advantages of CFRP over steel, namely its light weight and corrosion resistance,

should enable the material to be used for niche applications such as in offshore environments.

When used as a replacement for steel, CFRP bars could be used to reinforce concrete structures,

however the applications are not common.

4.8 CLOSING REMARKS

It can be concluded that recent developments in the field of FRPs have resulted in its use as a

highly efficient construction material. CFRPs are being used increasingly to rehabilitate

corrosion affected structures. CFRP wraps provide a barrier layer that impedes further corrosion

of steel and prevents the increase in volume of RC members due to rusting by applying

confinement pressure, thereby preventing dislodging of concrete cover.


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CHAPTER 5

LITERATURE REVIEW

5.1 GENERAL

This chapter presents a thorough review of literature on nondestructive monitoring techniques as

well as the Ultrasonic guided waves for monitoring rebar corrosion in concrete and it is observed

that corrosion monitoring is necessary as it allows the corrosion to be caught before its onset and

opens up much more economical maintenance options such as coating, retrofitting etc. Also a

thorough review of literature on Fiber Reinforced Plastic (FRP) composite wraps for corrosion

protection is presented as FRP wraps have proved to slow down the rate of corrosion and

prevents the structure from corrosion damage in chloride contaminated concrete.

5.2 LITERATURE REVIEW ON ELECTROCHEMICAL TECHNIQUES

Angst et al. (2009) studied linear polarization method (LPR) and Electrochemical impedance

spectroscopy (EIS) for detecting active corrosion and stated that the most accurate technique to

detect depassivation of the steel is the measurement of the linear polarization resistance, which is

inversely proportional with the corrosion current as described by the SternGeary-equation. By

applying the SternGeary-equation and assuming Tafel slopes the corrosion rate can then be

calculated.

Bhattacharjee et al. (2009) illustrated the findings of an experimental investigation carried out

on large number of specimens for evaluating the performance of different types of rebar in

chloride contaminated concrete made with different types of cement through different corrosion

rate techniques. Corrosion rate were obtained by linear polarization resistance (LPR) technique

with guard ring arrangement and AC impedance spectroscopy.

Bhavneet et al. (2010) used two non-destructive techniques namely electrochemical LPRmeasurement and ultrasonic guided waves for determining the corrosion initiation and

progression. It was concluded that electrochemical measurements are effective only in

monitoring corrosion initiation and are not useful to measure corrosion progression whereas

ultrasonic guided waves are effective for both monitoring and progression of corrosion.


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Broomfield et al. (2002) appliedcorrosion monitoring techniques to monitor durability in new

constructions, and as well as on existing structures to evaluate rehabilitation strategies such as

corrosion inhibitor applications and patch repairs. Corrosion monitoring systems include linear

polarization, concrete resistivity and half cell potential measurements. The author concluded that

this monitoring system is very effective to access the condition of new and rehabilitated

structures well in time.

Goueygou et al. (2008) experimentally studied the effectiveness of two non-destructive testing

techniques: the measurement of electrical resistivity and the transmission of ultrasonic surface

waves, to detect the width and depth of crack pattern. Rectangular concrete slabs reinforced with

10 mm bar and 25 mm cover the test specimens were cast. A major crack was induced in the

middle of the specimen using a three point bending setup. Electrical resistivity measurements

were made with a four probe square device and Ultrasonic Rayleigh wave technique was used to

measure pulse velocity, phase velocity and peak attenuation. The test results showed that, both

techniques were able to detect the presence of a main crack in the middle of the specimen but the

secondary cracks around the main crack and the depth of the crack were detected only by

ultrasonic technique.

Jung et al. (2002) investigated the feasibility of detecting internal defects (cracks, honeycombs,

inclusions) in reinforced concrete beams using ultrasonic guided waves (Lamb waves).

Experiments were carried out on full-scale beam specimens. The experimental results showed

that it Lamb waves were able to detect the presence of cracks because in lamb wave method the

defect detection does not depend on the reflection of waves from defects but on how the waves

interact with them. The author also concluded that the lamb wave technique gives stronger

differences in ultrasonic signal strengths from defective and defective free regions in comparison

to the conventional ultrasonic methods.

Koleva et al. (2006) tested small scale cylindrical RC specimen to investigate the corrosion

behaviour of steel in concrete. Steady direct current was applied to reinforcement steel (cathode)

for corrosion prevention and protection. A cylindrical titanium mesh served as counter electrode

and saturated calomel electrode (SCE) as reference electrode. The specimens were monitored

using LPR and EIS techniques. Tafel plots were used to find the electrochemical parameters such

as polarization resistance (Rp), corrosion potential (Ecorr) and corrosion current density (Icorr).


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The corrosion current density was estimated by SternGeary equation. Test results indicated that

the corrosion current density decreased significantly for protected specimens.

Li et al. (2006) studied the application of steel thin film electrical resistance (TFER) sensor for

in situ corrosion monitoring. For this, the sensing element of the sensor was designed to have a

multiple-line pattern, in order to study the sensitivity of TFER to various corrosive

environments, and the feasibility to detect the localized feature of corrosion. The polarization

resistance (Rp) of thin films and bulk steel were measure

ultrasonic monitoring of concrete

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